1. Field of the Invention
The present invention relates in general to a radar system using a quadrature signal. More specifically, an exemplary embodiment of the present invention relates to a low-price, high-sensitivity, high-performance subminiature radar system using a quadrature signal, wherein the radar system has many advantages in that a quadrature push—push oscillator composed of low frequency elements is employed to offset a leakage signal of a sending end and thus, increases receiving sensitivity; an increase in a noise figure of a receiving end caused by the leakage power can be prevented; the size of the radar system can be reduced markedly by sharing a transmitting and receiving antenna; a quadrature subharmonic mixer for the receiving end is used to suppress the occurrence of a DC-offset, which is resulted from even order distortion due to interference (or jamming) and self-mixing of a mixer due to signal leakage of a local oscillator; and the directional information on a target object, e.g., whether the target object is getting closer or getting away, can be obtained.
2. Description of the Related Art
In general, a vehicle collision warning radar system operates in 77 GHz (millimeter wave) band. However, making a signal source in such frequency band is very difficult and requires high-performance devices, which in turn increases costs of manufacture.
FIGS. 1A and 1B are circuit diagrams of a related art radar system. Particularly, FIG. 1A illustrates a radar system using a single antenna for transmitting and receiving, while FIG. 1B illustrates a radar system using two separate antennas for transmitting and receiving.
As aforementioned, the radar system in FIG. 1A uses a single antenna 104 so the size of the entire system can be reduced. However, the isolation between the sending signal and the receiving signal for the circulator 201 and the power divider 102 is extremely low in a millimeter frequency band. Thus, if a signal flows from a sending end with high power output into a receiving end, the receiving sensitivity of the radar system becomes so low that the radar system cannot detect a weak received signal.
In addition, if a high power signal flows into the receiving end, then a low noise amplifier 105 or a mixer 106 of the receiving end may become saturated, thereby deteriorating the entire system characteristics.
Also, because of the leakage signal of the sending end the noise figure of the receiving end gets worse.
The above mentioned problems are solved by developing a high-output radar system which uses two separate antennas as in FIG. 1B for transmitting and receiving.
In case of using two separate antennas, i.e., a transmitting antenna 104a and a receiving antenna 104b, the isolation between the sending end and the receiving end outperformed a case of the circulator 201. Thus, the receiving sensitivity of the system is improved and the saturation problem with the receiver is prevented. Nevertheless, these advantages do not necessarily outweigh the problem caused by using two antennas 104. That is, the area occupied by using two antennas 104 in the radar system is too large to embody a subminiature radar system.
In case of a CW (Continuous Wave) radar, a DC-offset occurs due to the even order distortion present in a direct conversion receiver and the leakage signal of a local oscillator. Moreover, in case of a Doppler radar, the Doppler shift is extremely low between tens of Hz to hundreds of kHz, so the radar is under heavy influence of flicker noise and therefore the receiving sensitivity of the radar is markedly deteriorated.
The quadrature mixer might be the optimal choice for obtaining directional information on a target object, i.e., whether a target object is getting closer or farther away, if an additional circuit for generating an additional phase difference in need of the quadrature signal generation for driving the mixer would not be required.